1. Field of the Invention
The present invention relates to aluminum/alumina composite materials and to methods for increasing the wettability of alumina fibers by molten aluminum or an aluminum alloy. More particularly, the present invention relates to improving the bonding at the interfaces between alumina fibers and aluminum metal.
2. Description of the Prior Art
The main difficulty encountered in forming alumina/ aluminum metal matrix composites (MMCs) is that molten aluminum and its alloys do not readily wet alumina. The wetting of the fibers by molten metal is critical to the formation of useful MMCs.
The function of the fibers in composites is to increase the strength and the fracture toughness. These two functions impose contradictory requirements on the alumina/aluminum interface. High strength is achieved through good load transfer from the matrix to the fibers and it requires strong bonding at the interface. High fracture toughness is achieved through crack energy dissipation.
In the past, it has been proposed to manufacture composite materials which comprise reinforcing-alumina fibers enclosed in an aluminum metal matrix by impregnating a suitable assembly of fibers with the molten metal. Alumina fibers do not exhibit strong reactivity with aluminum alloys and therefore they can be used as a compatible reinforcement material. In addition, alumina is very refractory (melting temperature is 1999.degree.C.-2032.degree.C.) and are capable of withstanding processing temperatures of molten aluminum.
It is desired that the alumina fibers be impregnated with the molten aluminum metal either through capillary action in which the fibers are partially or wholly immersed in the molten metal which can be aided by vacuum action in which the fibers are enclosed in an evacuated chamber and the molten metal is admitted into the chamber.
Incomplete wetting of the alumina fibers and the aluminum metal matrix creates voids within the resultant composite material which, in turn, weakens the composite. Then too, even if acceptable wetting is achieved, sufficient bond strength at the interface between the fiber and the metals is desired to obtain high strength. Additional difficulties may be encountered during subsequent welding or brazing of the composite materials. Localized melting of the metal matrix composite during welding or brazing operations may cause a corresponding localized de-wetting of the reinforcing fiber. This, in turn, leads to porosity in the region of the weld or braze. Generally speaking, ineffective wetting and/or ineffective bonding at the interface between the composite layers will degrade the composite's properties.
Additionally, the impregnation of the fibers with the molten metal can take a long time, thereby possibly causing deleterious interaction between the fibers and the metal.
Various prior art attempts to address these problems are known. However, the prior art methods each suffer from one or more serious drawbacks making such methods less than entirely suitable for their intended purposes.
For example, pressure infiltration or squeeze-casting, forcing the molten aluminum into the fibers by employing high pressure, is a common prior art technique. Grimshaw et al U.S. Pat. No. 4,232,091, issued Nov. 4, 1980, describes applying at least 75 kilograms per square centimeter to overcome the surface tension between alumina fibers and molten aluminum or alloys of aluminum, thereby hoping to assure penetration. Donomoto et al U.S. Pat. No. 4,450,207, issued May 22, 1984, found, furthermore, that pressurization at approximately 1,000 kilograms per square centimeter was required in order to infiltrate molten aluminum matrix metals (even having from 0.5 to 4.5 weight percent magnesium for optimum bonding characteristics) into interstices of the reinforcing alumina fibers. In practice, these processes have tended to cause channeling in the mold and thereby do not often assure optimum contact between a metal and the fibers.
Additionally, lithium doping of aluminum and aluminum alloys has been proposed to increase the wetting of the alloy on the alumina fibers. A reaction occurs between the lithium and the alloy at the surface of the fibers which surface becomes gray to black due to the formation of lithium aluminate. For example, Riewald et al U.S. Pat. Nos. 4,012,204 and 4,053,011, issued Mar. 15, 1977, and Oct. 11, 1977, respectively describe composite materials comprising an aluminum-lithium matrix, reinforced with polycrystalline alumina fibers. In order to retain useful strength in the fibers, not more than 15% of the total diameter of the fiber should react with the lithium. Accordingly, the reaction conditions must be carefully controlled insofar as initial lithium content, temperature, and particularly pressure are concerned. In fact, a pressure differential of about 2 to 14 pounds per square inch was needed to overcome the molten metal's resistance to penetration into the alumina fibers.
As previously mentioned, substantial oxidation at the interface between alumina fiber and aluminum metal has been found to be detrimental. Recently, such a problem associated with the presence of oxides at the surface of an aluminum metal layer was addressed by Weiss et al U.S. Pat. No. 4,687,043, issued Aug. 18, 1987. Therein a zinc solder alloy was used interfacially to protect an alumina fiber surface at the interface between aluminum layers to be cast thereon.
Accordingly, it is a principal objective of the present invention to provide a method for improved bonding of aluminum metal or alloys to refractory fibers wherein the fibers are "wet" by the aluminum or aluminum alloy.
Another object of the invention is to provide an alumina fiber reinforced aluminum alloy system which exhibits good interfacial wetting that does not require a low partial pressure of oxygen during fabrication.
A further objective of the present invention is to provide a method that does not require an excessive pressure differential to force fiber impregnation of molten metal.
Yet another object of the invention is to provide an alumina fiber reinforced aluminum alloy system which exhibits good interfacial wetting without the need to use reactive metals such as lithium as a wetting agent.
Yet another object of the present invention is to improve the strength of an alumina fiber/aluminum alloy MMC by increasing bond strength at the interface.
These and other objects and advantages of the present invention will be more fully understood and appreciated with reference to the following description.